In one typical system for continuously casting a molten substance, an endless flexible metal band is guided about one or more generally fixed position idler or tension band wheels and a portion of the peripheral groove of a casting wheel rotatable about a fixed axis. For the purpose of example but not limitation, the casting of molten metals such as copper, steel, and aluminum, or alloys thereof, will be discussed. The molten metal to be cast is poured in a continuous manner into the moving mold portion of the casting apparatus formed by the band and the peripheral groove in the casting wheel as the casting wheel is rotated by an external drive mechanism. Coolant is applied to the external and/or internal surfaces of the wheel and to the outside surface of that portion of the band which closes the peripheral groove of the casting wheel, said coolant acting functionally to extract the heat from the molten metal at a rapid rate and to prevent the casting wheel and band from overheating. At the time the metal band is removed from the peripheral groove of the casting wheel by the band guide wheels the previously molten metal is sufficiently solidified to permit extraction from the casting wheel and be guided on to a succeeding stage in the continuous casting process.
A key component in the operation of such wheel-band molten metal casting apparatus is the band. Indeed, economies created by the wheel-band machines in the past which have reduced the cost of the manufactured rod/strip/wire product while increasing product quality are largely attributable to the inherent continuousness of operation of such machines. Once wheel life is maximized well beyond band life, band life becomes a factor most limiting the continuousness of operation of the casting machine. Extending band life therefore functions to reduce operating down time, maintenance time, and operating and maintenance costs. Other systemic advantages accompany extended band life, including longer component lifetimes for thermally cycled system parts (pots, launders, pour spouts, casting wheels, furnaces, and burners), more energy efficient casting, greater overall productivity, reduced scrap product, longer life of subsequent mill rolls (due to fewer startups), and a better quality product.
In the operation of wheel-band continuous casting systems of the type generally described above, one of the major problems is the care, maintenance, and replacement of the band. Due to the need to form the band into arcs to pass around the casting wheel and band guide wheels, it must be made of flexible materials. Additionally, thinner bands permit more efficient transfer of heat from the molten metal than thick bands while having only two drawbacks; first, thin bands are more subject to band tensioning difficulties, and second, thin bands undergo more strenuous thermal cycling, compounding any tension difficulty tendencies. The two most common failure modes are due to thermal and mechanical stresses; both effects must be carefully considered when selecting materials for casting bands. Other factors to be considered include band cost, cost of preparation, ease of installation, band life, and heat extraction efficiency, the latter being of special import in the casting of alloys containing elements of differing solidification temperatures.
Generally, bands for this type of casting machine have been selected from among very low carbon steel alloys and copper and copper alloys when casting molten metals. One low carbon steel alloy in common use is A.I.S.I. 1006 or 1008 grade, having a good tensile strength (40,000 to 60,000 psi), low linear expansion, easily joined ends (TIG welding proves durable), is low in cost and is characterized by numerous other advantages when used for casting bands. Often, however, when bands of this material are improperly tensioned, they have been further characterized by short operating lifetimes, relatively low thermal efficiency, and a tendency to distort the draft angle of the casting wheel when under excess tension, thereby resulting in difficulty in extracting the cast bar and thus requiring early replacement of the quite expensive casting wheel.
Copper and certain alloys of copper have been tried as casting bands, with some successes noted. The primary advantages of such bands are substantially equal thermal expansion factors, improved thermal conductivity, and under certain tensioning arrangements, claimed longer band life.
U.S. Pat. No. 3,938,580 in part addresses itself to these latter factors. Indeed, a major advantage given in the cited specification is directed to the advantages obtained through the use of bands of such materials. While the embodiments of U.S. Pat. No. 3,938,580 are directed, in part, to the application of a controlled tension to an expansible (copper or copper alloy) band and consequently requires a significantly long slidable tensioning apparatus to suitably entrain the moving, expanding band, certain inherent limitations of copper and copper alloy bands reduce the effectiveness of the apparatus of said patent in producing a less costly or improved product.
It is known in the wheel-band type continuous casting art that certain copper alloy bands, under certain conditions, may appear to perform better than conventional low carbon steel alloy bands. These bands provide both improved thermal conductivity for extracting heat from the solidifying cast bar and thermal expansion ratios that are more equal to those of common casting wheels. However, bands made from these alloys are initially substantially more expensive than the normal low carbon steel bands; they are significantly more difficult to join into a continuous band in that special, expensive, and more critical welding procedures are required; and yet experience has shown that they frequently last no longer than perhaps twice as many hours as do steel bands. It should be noted that the band joining problem with copper or copper alloy bands is not only expensive, exacting, and difficult; but should the weld be less than perfect, and should the band separate suddenly due to a poor weld joint, operating personnel may be exposed to calamitous molten metal spills and resultant explosions. Such calamitous molten metal spills are all the more likely when using a copper or copper alloy band, as opposed to a low carbon steel band, because the weld joint, often one of the weakest points in the band, is exposed to mechanical stress from bending arcuately around the idler or casting wheel at nearly the same instant it is subjected to severe thermal shock from contact with the molten metal.
Listed as an advantage in the specification of the referenced patent, the linear expansion factor of copper and copper alloy bands also results in a cooling problem, though of less importance than that would be a calamitous molten metal spill. As the band stretches under the "constant" tension of the apparatus of U.S. Pat. No. 3,938,580, it becomes increasingly thin. Experience has shown that thinner bands dissipate extracted heat more rapidly with a given coolant flow. With no method or apparatus disclosed directed to continuous alteration of the volume of coolant flow directed to the band, more heat will be extracted sooner from the solidifying metal as the band becomes thinner; unless alterations in the coolant flow are made, changes in the crystalline structure of the cast bar may occur with long casting runs, with concomitant changes in the characteristics of the rod rolled therefrom. Further, so-called "ballooning" of the band may occur, wherein the band (as seen together with the wheel in cross section) withdraws diametrically outwards at its center from the molten metal in the casting wheel groove. Ballooning reduces the cooling ability of the band and at the same time increases the likelihood of a molten metal spill due to more limited contact of the band edges with the wheel. It is well known in the art that the probability of such ballooning increases as the band is thermally and mechanically cycled and stretched.
Copper and copper alloy casting machine band expansions are not entirely linear, as has been suggested in the prior art. Experience has shown that the expansion factor does for some time remain approximately linear, until reaching a certain determinable point, after which further uniform elongation is greatly diminished and after which time band failure due to overstretching becomes more likely with the progression of time. With copper or copper alloy bands the greatest share of band operating time occurs in this non-linear expansion portion of the band life time, during any portion of which the band may fail with little or no warning.
In order to determine the optimum tension applied to a given band length/material/casting machine combination, it is necessary to easily and accurately determine the tension applied to enable recording of the different tension information. An involved mathematical formula may be used to calculate this tension, as with the apparatus of U.S. Pat. No. 3,938,580, or a simple formula may be derived should the apparatus of the present invention be used. In the referenced patent, assuming all forces to be in a common vertical plane, it should be understood that as the angle of linear motion referenced to horizontal is changed (generally decreased in the referenced patent) and concomitantly the angle of one or more segments of the band with reference to the line of action of the retreating tensioning wheel decreases, the band tension forces change non-linearly. These and other factors introduce undue complexity in the determination of the band tension applied.
The general formula for determining the tension applied to a band is given as formula I; this formula is required in calculating the tension for tension means such as are presented in the embodiments of the cited patent.
The following symbols and definitions are used in formula I and associated FIG. 10:
Weight of band, which is small, is ignored.
P=Fluid pressure (Gauge)
A=Net piston area
F=Force exerted by cylinder
W=Weight of slide, wheel & other moving parts acting through the center of gravity.
T=Tension force in the band
.alpha.=Angle of linear motion to horizontal
.beta.=Angle of one segment of band with line of action
.delta.=Angle of 2nd segment of band with line of action
Po=Fluid pressure at rest without band (pressure to overcome resultant of W)
Assuming that both sides of the piston and cylinder are at atmospheric pressure, neglecting friction and the weight of the band, and assuming that all forces are acting in a single plane: